Numerous white diffusely reflecting materials and coatings have been developed for use in radiometric calibration procedures and in bidirectional reflectance measurements, such as those employed as part of various remote sensing studies. These materials and coatings have included magnesium oxide, magnesium carbonate, barium sulfate, sodium chloride, sulfur and aluminum oxide. Coatings including homopolymers and copolymers with a substantial degree of fluorine substitution have also been used for this purpose because of the highly reflective properties of the polymers. Magnesium oxide, when freshly smoked, exhibits excellent average diffuse reflectance and magnitude of reflectance but it suffers from atmospheric instability and fragility.
In one instance, polytetrafluoroethylene (TFE), functioning as a pigment, was dispersed in a methyl acetate concentrate of a vinyl acetate-alcohol copolymer binder which was diluted for heavy loading of TFE by use of ethyl alcohol. The ethyl alcohol served as a low residue vehicle. This binder for the TFE proved to be generally too soft and also to be an inferior adhesive. Moreover, after application, the binder softened whenever the coating was subjected to water and did not re-harden until it re-dried.
Of all these diffusely reflecting materials and coatings barium sulfate has achieved the broadest scientific and commercial acceptance. The coating is prepared by having the barium sulfate, the pigment, weakly tethered with a water-soluble form of a polyvinyl alcohol (PVA), the binder, and water. The resulting mixture, however, is subject to phase agglomeration, i.e., because of interactions between the pigment and binder, the barium sulphate and PVA tend to settle to the bottom of the storage container with time and the container must be shaken severely to achieve a reasonable mix of the constituents. Also, the applied coating, while it is highly reflective and exhibits a Lambertian reflection characteristic, is very soft, water soluble and unwashable. Therefore, upon soiling, its optical properties cannot be restored. Further, it gradually changes its optical properties in sunlight. In a later development, ethyl alcohol was added to the barium sulphate-PVA-water mixture to substantially alleviate the phase agglomeration problem. The problems relating to its change in optical properties in sunlight, coating softness, water solubility and non-washability, however, remain.